John Corvino has done an excellent series of videos addressing the arguments against marriage equality, which you can find here. They are concise, clear, and rational, and go a long way toward illustrating just how empty those arguments are.
Here's the one on marriage as a threat to religious freedom:
My one objection to this one is that I think he should have stressed the fact that clergy who perform weddings do so as agents of the state -- their authority is secular, not religious.
And here's the Plus: one of the clearest and most sensible opinions on the limits of "religious freedom" I've seen from anyone. Via Think Progress:
The burden of which plaintiffs complain is that funds, which plaintiffs will contribute to a group health plan, might, after a series of independent decisions by health care providers and patients covered by [an employer's health] plan, subsidize someone else’s participation in an activity that is condemned by plaintiffs’ religion. . . . [Federal religious freedom law] is a shield, not a sword. It protects individuals from substantial burdens on religious exercise that occur when the government coerces action one’s religion forbids, or forbids action one’s religion requires; it is not a means to force one’s religious practices upon others. [It] does not protect against the slight burden on religious exercise that arises when one’s money circuitously flows to support the conduct of other free-exercise-wielding individuals who hold religious beliefs that differ from one’s own. . . .
[T]he health care plan will offend plaintiffs’ religious beliefs only if an  employee (or covered family member) makes an independent decision to use the plan to cover counseling related to or the purchase of contraceptives. Already, [plaintiffs] pay salaries to their employees—money the employees may use to purchase contraceptives or to contribute to a religious organization. By comparison, the contribution to a health care plan has no more than a de minimus impact on the plaintiff’s religious beliefs than paying salaries and other benefits to employees.
Here's the full decision.